1. Field of the Invention
The present invention relates to a liquid crystal display device having a wide viewing angle property and exhibiting excellent color reproducibility over a wide viewing angle. Also, the present invention relates to improvements of color reproducibility and contrast viewing angle of a liquid crystal display device, particularly, an IPS-mode liquid crystal display device.
2. Description of the Related Art
Display devices using a liquid crystal display element (sometimes called a liquid crystal display panel), an electroluminescent element (classified into an organic system and an inorganic system by the fluorescent material used; hereinafter referred to as “EL element”), a field emission device (hereinafter referred to as “FED element”), an electrophoretic element or the like perform the image display without providing a space (vacuum casing) for two-dimensionally scanning electron beams, such as cathode ray tube (CRT), behind the display screen. Accordingly, these display devices are characterized by being thin and lightweight and less consuming the electric power. Such a display device is sometimes called a flat panel display from its characteristic feature in appearance.
Because of those advantages over the cathode ray tube, the display device using a liquid crystal display element, an EL element, a field emission element or the like is widely spreading in various uses such as OA equipment (e.g., notebook computer, monitor for personal computer), portable terminal and television, in place of the display device using a cathode ray tube. Behind the progress of shifting to the flat panel display from the cathode ray tube, there is a technical innovation such as enhancement of the pictorial quality, for example, enlargement of the viewing angle property or display color reproduction region of a liquid crystal display element, an EL element and the like. Also, with recent spread of the multimedia or Internet, the movie display performance is enhanced. Furthermore, expansion into the fields unrealizable by CRT, such as electron paper and large-screen information display for public use or advertisement, is proceeding.
The liquid crystal display device usually comprises a liquid crystal cell, a driving circuit for feeding a display signal voltage to the liquid crystal cell, a backlight (light source on the back side) and a signal control system for sending an input image signal to the driving circuit, and these are collectively called a liquid crystal module.
The liquid crystal cell usually comprises a liquid crystal molecule, two substrates for encapsulating and sandwiching the liquid crystal molecule, and an electrode layer for applying a voltage to the liquid crystal molecule, and a polarizing plate is further disposed outside of the liquid crystal cell. The polarizing plate usually comprises a protective film and a polarizing film and is obtained by dyeing a polarizing film comprising a polyvinyl alcohol film with iodine and after stretching the film, stacking a protective film on both surfaces thereof. In a transmissive liquid crystal display device, in some cases, this polarizing plate is fixed to both sides of the liquid crystal cell and one or more optically compensatory sheets are further disposed. Also, in a reflective liquid crystal display device, a reflector plate, a liquid crystal cell, one or more optically compensatory sheets, and a polarizing plate are usually disposed in this order. The liquid crystal cell performs the ON-OFF display by the difference in the orientation state of liquid crystal molecules and is applicable to any of the transmissive, reflective and transflective liquid crystal display devices.
When an optically compensatory sheet with the retardation values in the plane surface and in the thickness direction being controlled is used, a liquid crystal display device reduced in the change of brightness dependent on the viewing angle can be provided. For example, JP-A-11-305217 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes, for the purpose of preventing light leakage when viewed from an oblique direction, a wide viewing angle polarizing film having a polymer orientation film of which in-plane retardation and retardation in the thickness direction satisfy a predetermined relationship.
Also, JP-A-2006-30937 discloses a film where the retardation values in the plane surface and in the thickness direction of the protective film of the polarizing plate are controlled, and proposes a liquid crystal display device reduced in the light leakage when viewed from an oblique direction by combining the film with an optically compensatory sheet.